The long-term objective of this work is to provide a description of the neuronal elements of the mammalian auditory system, together with knowledge of their organization and function. One of the approaches toward this objective is to identify distinctive neuronal cell classes by their characteristic cytochemical properties, as revealed by immunocytochemistry. Connections of immunocytochemically identified cells are demonstrated by combining pathway tracing experiments with immunostaining. Such results reveal far greater concerning connections of individual cell types than would otherwise be possible. Antibodies that prove useful for analyzing animal tissue are applied to human material to learn the extent to which animal and human tissue are similar and to provide a basis for inferring the organization of the human system. The proposed work focuses on the organization of the cochlear nucleus (CN) and its connection. Immunocytochemistry will be used to demonstrate characteristic cytochemical constituents of stellate cells and of axon terminals on them. Unique chemical signatures will be used to distinguish various stellate subclasses. The projections of chemically distinct CN cell classes will be determined by injecting horseradish peroxidase (HRP) into the inferior colliculus and lateral lemniscal nuclei of cats to retrogradely label CN neurons and then immunostaining the labeled CN neurons to show their characteristic chemical signatures. Inputs to the CN will be similarly identified by injecting HRP into the CN and then immunostaining the retrogradely labeled cells within the superior olive and the inferior colliculus to learn what neuropeptides or neurotransmitter-related enzymes are found within ells providing CN inputs. Issues concerning the presence and numbers of CN octopus cells and principal cells of the medial nucleus of the trapezoid body in humans will be addressed using antibodies that characterize these cells in cats. Results should produce considerable new information about the composition and organization of brainstem auditory structures in cats and humans. Such information will ultimately be useful for understanding the role of these structures in hearing and in better understanding the mature of deficits that occur when these structures are diseased or damaged.